Electron beam instrumentation for FELs

Frontier fast dynamics science is strongly founded on accelerator-based pulsed photon sources. The longitudinal electron bunch properties play a crucial role to guarantee high performance of the most advanced accelerator-based facilities. Advanced instrumentation have been designed to control picosecond and sub picosecond electron bunch duration. The simplest and most robust bunch length diagnostics are based on the measurement of coherent radiation power and they are used in existing accelerators to measure the relative variation of the bunch length non-destructively and shot by shot. The resulting information is used in bunch length control feedback loops. The drawback is that for an absolute estimation of the bunch length, external instrumentation like a transverse RF deflecting cavity is usually needed.
We developed a novel experimental methodology to self-calibrate a simple device based on diffraction radiation from a ceramic gap and demonstrated its capability to provide the second order moment of the electron bunch longitudinal distribution (i.e. the bunch length). The method is best suited for measuring bunches with durations from picoseconds to sub picoseconds. Moreover it does not depende upon other external  instruments and the required equipment is extremely simple.
In general incoherent radiations have been used for a variety of electron diagnostics but they do not provide any information on the bunch as a whole since each electron in the bunch emits radiation with a different phase. Electrons in a very short bunch emit radiation, with wavelength of the order of the bunch length, with the same phase. This is called coherent radiation. Since all these electromagnetic waves have the same phase the sum of the amplitudes adds as N², where N is of the order of 10 billion electrons per bunch. Consequently the coherent emission is very intense, increasing inversely with the bunch length. The spectral content of the coherent radiation of a picosecond – sub picoseconds electron bunch is enhanced in the millimeter waves and THz spectrum range.
In order to provide an absolute bunch length measurement by using the coherent emission of the bunch we need other important ingredients. The first one is finding an analytical formalism for describing the physics of the system. In our device we detect the diffracted coherent radiation emitted by the electron bunch passing through a ceramic gap. This Maxwell problem has a formal analytical solution which is very complex and hardly usable for practical applications. For this reason we developed an analytical approximation that can be used to calculate the bunch length. The second ingredient is related to the spectral content. This depends on the Fourier transform of the longitudinal bunch distribution. However, by choosing a properly low detection frequency and using a relatively small bandwidth we can measure only the bunch length dependence without being affected by the bunch shape details. The third ingredient is a distinctive feature of the coherent emission from a gap in a waveguide: as the bunch gets shorter the emitted signal intensity initially increases but for very short bunches it reaches an asymptotic value.